Showing papers on "Norbornadiene published in 2000"

Synthesis and Properties of Dinuclear Complexes with a Photochromic Bridge: An Intervalence Electron Transfer Switching “On” and “Off”

Abstract: Two cyclometallated Ru(bpy)2(pp) units (bpy = 2,2′-bipyridine; pp = 2-phenylpyridine) were grafted via ethynyl spacers onto the photochromic cores of the norbornadiene or dithienylethene families, to give the bimetallic ruthenium(II) complexes 1 and 2a, respectively. Two aspects of these compounds were studied: (i) photoisomerization (norbornadiene to quadricyclane for 1, open form to closed form of the dithienylethene core for 2a), and (ii) intervalence transitions in the mixed-valence ruthenium(II−III) state, with determination of the metal−metal electronic coupling (Vab) by Hush’s equation. Moderate metal−metal electronic coupling (0.068 eV) was found for 1, but 1 cannot be isomerized into the quadricyclane form. On the other hand, 2a can be reversibly photoisomerized to the closed form 2b. An iodine(III) reagent, the triflate salt of bis(pyridyl)phenyliodonium, was used for the oxidation of 2a and 2b. During these oxidations, an intervalence band due to intramolecular electron transfer between ruthenium(II) and ruthenium(III) was detected for 2b, but not for 2a. This switching “on” or “off” of the intervalence transition is rationalized with extended Huckel calculations. Finally, the molecular orbital calculation explains the observed tendency of the closed form 2b to reopen upon oxidation.

Molybdenum(0), ruthenium(II), palladium(II), platinum(II), copper(I) and gold (I) complexes of a new methoxy functionalised bis(phosphino)amine: synthesis and structure

Abstract: The synthesis, and characterisation, of a new functionalised bis(phosphino)amine Ph2PN(o-C6H4OMe)PPh21 from o-H2NC6H4OMe and 2 equiv. of Ph2PCl in diethyl ether at 0°C is reported. Oxidation of 1 with either aqueous H2O2, elemental S8 or grey Se affords the phosphorus(V) compounds Ph2P(E)N(o-C6H4OMe)P(E)Ph2 (E=O 2; S 3 or Se 4). Partial oxidation of 1 with 1 equiv. of S8 in n-hexane affords the mixed P(III)/P(V) species Ph2P(S)N(o-C6H4OMe)PPh25 in addition to small amounts of 3. Reaction of 1 (or 5) with [MX2(cod)] (M=Pd, Pt; X=Cl or CH3; cod=cycloocta-1,5-diene) affords either cis-[MX2{Ph2PN(o-C6H4OMe)PPh2}] (M=Pd, X=Cl 6; M=Pt, X=Cl 7; M=Pt, X=CH38) or the neutral five-membered chelate complexes [MCl2{Ph2P(S)N(o-C6H4OMe)PPh2}] (M=Pd 9, M=Pt 10) in which P,P- or P,S-chelation respectively was observed. Likewise reaction of [Mo(CO)4(nbd)] (nbd=norbornadiene) or [Cu(MeCN)4]PF6 with 1 gave cis-[Mo(CO)4{Ph2PN(o-C6H4OMe)PPh2}] 11 or [Cu{Ph2PN(o-C6H4OMe)PPh2}2]PF612. Chloro-bridge cleavage and arene elimination of [{RuCl2(p-cym)}2] (p-cym=p-cymene) with 1 (1:2 metal:ligand ratio) affords, in good yield, the octahedral bis chelate ruthenium(II) complex trans-[RuCl2{Ph2PN(o-C6H4OMe)PPh2}2] 13. In contrast, reaction of 1 with two equiv. of [AuCl(tht)] (tht=tetrahydrothiophene) gave the dinuclear complex [(ClAu)Ph2PN(o-C6H4OMe)PPh2(AuCl)] 14 in which the bis(phosphino)amine P,P-bridges two {AuCl} metal fragments. All new compounds have been characterised by a combination of multinuclear NMR [1H, 31P{1H} and 195Pt{1H}], IR spectroscopy and elemental analyses. The molecular structures of six representative examples have been determined by single-crystal X-ray crystallography.

Characteristics of RuCl2(CHPh)(PCy3)2 as a Catalyst for Ring-Opening Metathesis Polymerization

Abstract: Ring-opened metathesis polymers and copolymers have been formed from norbornene, norbornadiene, a range of their derivatives, and cyclopentene using RuCl2(CHPh)(PCy3)2, as catalyst. 13C NMR analysis of the polymers illustrate a rather selective propagation reaction. The catalyst is highly regioselective, and the polymers are generally high trans with a strong isotactic bias. However, polymers from diene monomers tend to be less isotactic than those from the corresponding monoenes, and in the case of 7-methylnorbornadiene the polymer has an overall syndiotactic bias. A rate enhancing effect, noted previously, due to an oxygen atom proximate to the monomer double bond, is less pronounced than with other initiators. Catalyst activity, in the case of certain diene monomers, was shown to be monomer dependent and rate enhancements were also achieved using phenol as solvent. The results are interpreted in terms of the steric and electronic properties of both the catalyst and the monomers.

Low-energy pathway for pauson-khand reactions: synthesis and reactivity of dicobalt hexacarbonyl complexes of chiral ynamines

Abstract: A family of dicobalt hexacarbonyl complexes of 1-(dialkylamino)-2-(trimethysilyl)acetylenes (3a−f) derived both from achiral and chiral amines [a, morpholine; b, (S)-2-methoxymethylpyrrolidine; c, (2R,5R)-2,5-bis(methoxymethyl)pyrrolidine; d, (±)-trans-2,5-bis(benzyloxymethyl)pyrrolidine; e, (2R,5R)-2,5-dimethylpyrrolidine; f, (S)-(α-methylbenzyl)benzylamine] has been prepared by a one-stage process from dichloroacetylene. The methanolysis at room temperature of these complexes (MeOH/K2CO3) induces the selective cleavage of the carbon−silicon bond, leading to the thermally unstable terminal ynamine complexes 12a−f. The Pauson−Khand reaction of 12a−f with strained olefins (norbornadiene and norbornene) takes place at unprecedentedly low temperatures (−35 °C) in the absence of chemical promoters. Diastereoselectivities of up to 94:6 are recorded in these reactions with the ynamine complexes derived from C2 symmetrical chiral auxiliaries (12c,d). The high reactivity depicted by terminal ynamines in the Pauso...

Use of the seven-coordinate complexes [MXY(CO)3(NCMe)2] (M=Mo or W; X, Y=halide) and their derivatives in homogeneous catalysis

Abstract: The seven-coordinate halocarbonyl complexes, [MXY(CO) 3 (NCMe) 2 ] (M=Mo or W; X, Y=halide) and their derivatives have been investigated in several types of homogeneous catalytic reactions. These include, the alkene metathesis polymerisation of norbornadiene using either, [MX 2 (CO) 3 (NCMe) 2 ] (M=Mo or W;X=Br or I), [MI 2 (CO) 3 (L′- P , P ′)] {M=Mo or W; L′=MeC(CH 2 PPh 2 ) 3 } or [MXY(CO)(L or L′- P, P ′)(η 2 -RC 2 R′)] {X,Y=Cl, Br or I; L=PhP(CH 2 CH 2 PPh 2 ) 2 ; R,R′=Me, Ph}. The isolation and characterisation of a key intermediate, [WI 2 (CO) 2 (η 4 -nbd)] ( 1 ) (nbd=norbornadiene) is also described. The development of water soluble polymerisation catalysts using water-solubilising pyridine ligands, and the biphasic polymerisation of styrene using [WI 2 (CO) 3 (NC 5 H 4 CO 2 Na-4) 2 ], ZrCl 4 , sodium lauryl sulphate and sodium hydrogen phosphate is also discussed. The room temperature polymerisation of phenylacetylene using [WX 2 (CO) 3 (NCMe) 2 ] (X=Cl, I), [WI 2 (CO) 3 (NCMe)L] (L=PPh 3 , AsPh 3 ) or [WBr 2 (CO)(NCMe)(η 2 -HC 2 Ph) 2 ] ( 4 ) is also described, and a mechanism is proposed for these polymerisation reactions.

On the behavior of alpha,beta-unsaturated thioaldehydes and thioketones in the Diels-Alder reaction.

Abstract: Alpha,beta-unsaturated thioaldehydes and thioketones, R1CH=CH-C(=S)R2, are prepared in situ by the reaction between the corresponding carbonyl compounds and bis(dimethylaluminum) sulfide. These compounds undergo [4 + 2] self-dimerization reactions in which one molecule serves as the heterodiene component and the other as the dienophile to afford different types of dimeric products depending on the R1 and R2: 1,2-dithiin and 1,3-dithiin (R1 = R2 = H), 1,2-dithiin (R1 = Ph, R2 = H, CH3), or dihydrothiopyran (R1 = R2 = Ph). These differences in selectivity are explained on the basis of the relative energies evaluated by molecular orbital (MO) calculations at the DFT (density functional theory) level. The calculations show that in the dimerization reaction of thioacrolein (I), the head-to-tail (S-C-S bonded) dimers are kinetically more stable by about 5 kcal/mol but slightly thermodynamically unstable by about 2 kcal/mol than the head-to-head (S-S bonded) dimers. The calculations on thiocinnamaldehyde (IV) indicate that the dimerization reactions of phenyl-substituted alpha,beta-unsaturated thioaldehydes and thioketones are almost equally controlled by thermodynamic and kinetic factors. These unsaturated thiocarbonyl compounds also function as heterodienes (C=C-C=S) in the cycloaddition reaction with norbornadiene and as dienophiles (C=S) in the reaction with cyclopentadiene.

Novel photo-responsive organic spin systems: preparation and properties of norbornadienes and spiropyrans with TEMPO radical substituents

Abstract: Several photo-responsive norbornadiene as well as spiropyran derivatives with TEMPO radical substituents were prepared and their light-induced structural changes to the corresponding quadricyclanes or merocyanine dyes and reverse reactions to the starting materials were investigated. While the intermolecular magnetic interactions tend to be preserved in norbornadiene–quadricyclane systems in spite of the differences of their Weiss temperatures, a switching of the intermolecular magnetic interactions was observed in the spiropyran–merocyanine systems.

Concerning the synthesis and enantioselective rearrangements of episulfoxides

Abstract: A novel synthesis of episulfoxides having the norbornane skeleton is possible by use of a rhodium catalyst to effect SO transfer from trans-stilbene episulfoxide to norbornene or norbornadiene. Analogous Rh2(OAc)4 catalysed sulfur transfer to these alkenes is also possible using propylene sulfide as the sulfur source. These methods did not give useful yields of products with alternative types of alkene substrate. A novel type of chiral lithium amide base reaction, involving the rearrangement of certain types of symmetrical ring-fused episulfoxides, gives alkenyl sulfoxide products in up to 88% ee. The structures of the products, including absolute stereochemistry, were assigned based on X-ray crystal structure determinations. p

Synthesis and properties of helical aliphatic polyacetylene containing a cholesteryl group in the side chain

Abstract: Monosubstituted aliphatic acetylenes such as cholesteryl 3-butynyl carbonate (1a) and cholesteryl 6-heptynyl carbonate (1d) were synthesized and polymerized with a Rh(nbd)[C 6 H 5 ) 4 ] (nbd: norbornadiene) catalyst in tetrahydrofuran to give soluble polymers in almost quantitative yields. Poly(1a), which was found to be composed of a cis structure in the main chain by NMR analysis, showed intense circular dichroism (CD) in the UV/Vis region assigned to the π-π* transmission of the helical main chain induced by the cholesteryl side group, while poly (1d) contains both cis and trans structure in the main chain without showing significant CD. Poly(1a) was not deteriorated in air after a month, although the cis to trans isomerization was observed to take place by annealing, together with the intramolecular trimerization of the main chain.

Synthesis and Properties of Novel Metal‐Linked Pn Ligand Complexes

Abstract: The reaction of [CpCr(CO)3]2 with P4 in toluene at 90 °C for 5 h affords the triple-decker sandwich complex [CpCr(CO)(3-P3)(CpCr)2(,1:5:5-P5)] (1) as a new product, along with the previously described products with P2, P3, and P10 units. The structure of the paramagnetic complex 1 has been established by single-crystal X-ray diffraction methods. A novel complex [{CpCr(CO)2}2(,1:1:2-P2){Mo(CO)4}2{(,1:1:5:5-P5)(CpCr)2}] (2) has been obtained from the reaction of [{CpCr(CO)2}2(,2-P2)] with [Mo(CO)4(NBD)] (NBD = norbornadiene). The X-ray structure of 2 reveals a six-membered Mo2P4 ring combining a Cr2P2 tetrahedral core and a triple-decker sandwich Cr2P5 moiety. The EPR spectra of the paramagnetic (S = 1/2) complexes 1 and 2 are reported. A noticeable hyperfine interaction with two 31P nuclei is observed for 2.

Efficient Synthesis of Carbocyclic Nucleoside, (±)-Homocarbovir Via π-Allylpalladium Complex Formation from the Allyl N,N-Ditosylimide Substrate

Abstract: The synthesis of a carbovir analogue, (±)-homocarbovir (3) was achieved from norbornadiene (4) in seven steps and 27% overall yield. This route involves a Meinwald-type rearrangement, an acid-hydrolysis of N-tosyl bicyclic enamine 5, and a Pd(0)-catalyzed coupling reaction.

Diastereoselective addition of monoterpenic alcoholates and thiolates to 2,3-dicarbomethoxynorbornadiene

Abstract: Addition of excess lithium salts of (−)-menthol or (−)-borneol to 2,3-dicarbomethoxynorbornadiene 1 affords the transesterification–addition products 4 and 5 . The thiols derived from the same monoterpenes give only the addition products to the activated double bond, 6 and 7 . In all cases, the addition reaction is totally exo -selective with respect to the norbornadiene moiety and moderately selective in discriminating the si- and re -face of the double bond. Both diastereomeric products are formed as a mixture of endo–exo epimers at C3. The major addition product of (−)-menthol to 1 is crystalline, of known absolute configuration, and has potential as a precursor of chiral ligands.

High-level Computational Study of the Site-, Facial- and Stereoselectivities for the Diels-Alder Reaction Between o-Benzoquinone and Norbornadiene

Abstract: Ab initio and DFT quantum chemical calculations have been applied to a study of the Diels-Alder reaction of o-benzoquinone as diene and norbornadiene as dienophile. Transition states for the different reactions are located and activation energies estimated. The prefered exo-π-facial selectivity and exo,endo-stereoselectivity exhibited in this cycloaddition are readily predicted using RHF/3-21G or higher levels of calculations. Differences between experimentally observed results and calculations may be explained by the postulation of a second, nonconcerted biradical mechanism leading to formation of hetero Diels-Alder products.

Neighboring effect of pyrazole rings: regio- and stereoselective Wagner–Meerwein rearrangement in electrophilic addition reactions of norbornadiene-fused pyrazoles

Abstract: The electrophilic addition reactions of norbornadiene-fused pyrazoles with bromine or p-nitrobenzenesulfenyl chloride provided the skeletally rearranged adducts regio- and stereoselectively probably via the neighboring group participation of a pyrazole ring accompanied by the formation of a bridged pyrazolium ion.

Kinetic studies on the hydrosilylation of phenylacetylene with R3SiH (R3 = PhMe2, Ph2Me, Ph3, Et3) using bis(1,2‐diphenylphosphinoethane)norbornadiene rhodium(I) hexafluorophosphate as catalyst

Abstract: Product distribution and kinetic studies on the hydrosilylation of phenylacetylene by Ph3SiH, Ph2MeSiH, PhMe2SiH and Et3SiH were performed using bis-[1,2-diphenylphosphinoethane]norbornadienerhodium(I) hexafluorophosphate, 1, as catalyst. Pre-equilibration of the catalyst with the acetylene produced hydrosilylations, pre-equilibration with the silane did not. The catalyst showed a pronounced selectivity for cis-addition to form β-products, t-PhCH­CHSiR3, unlike most hydrosilylation catalysts. The kinetic studies showed a hydrosilylation reaction that is zero order with respect to both acetylene and the silane, with a dependency upon catalyst concentration. The kobs value is directly influenced by the substituents on the silane: k(PhMe2SiH) > k (Et3SiH > k (Ph2MeSiH) > k (Ph3SiH). Intercalation of the catalyst in hectorite was not useful, since either no reaction occurred in non-polar solvents, or extraction of the catalyst occurred in polar solvents to produce the same product distributions. Copyright © 2000 John Wiley & Sons, Ltd.

The anomalous course of the microsomal transformation of the exo-2,3-epoxides of norbornene and norbornadiene. The possible involvement of a general acid activation during the enzymatic hydrolysis of these oxides

Abstract: The enzymatic hydrolysis of exo-2,3-epoxy-norbornane ( 1 ) with a crude rabbit liver microsomal preparation occurred with a rearrangement and gave selectively (2 R ,7 S )-bicyclo[2.2.1]heptane-2,7-diol ( 3 ) , enantiomeric excess (ee) 30±2%. The analogous exo-2,3-epoxy-5-norbornene ( 2 ) gave, under the same conditions, exclusively endo-6-hydroxymethylbicyclo[3.1.0]hex-2-ene ( 4 ) , arising from the microsomal catalyzed reduction of the first formed endo-6-formylbicyclo[3.1.0]hex-2-ene ( 5 ) . A mechanistic explanation for the observed products is proposed.